FLOOD04447

• County: Fremont
• Community: Florence
• Stream Name: Oak Creek
• Basin: Arkansas
• Title: City of Florence LMMP Study
• Date: 2/1/1993
• Prepared For: FEMA
• Prepared By: Love & Associates, Inc.
• Floodplain - Doc Type: Floodplain Report/Masterplan

~~om Smith
Oak Creek Floodplain and Floodway - Florence, Colorado
FEMA LMMP Contract No. EMW-90-C-3132
Page 2

irrigatioa canal alignment. This combined flow of 3,44,(1 cfs continues east alcng the railroad to
the low point in the tracks at about Frazier Avenue. The flow spills continuously over the
railroad along this length of track.

Downst:eam of the D&RGW, the 100-year floodplain is delineated using three HEC-2 models:
main channel, right overbank area, aad left overbaak area. Thtee models are needed to define the
floodplain due to the split flow nature of the floodpl~in and the tOpography in the area. The flow
in the left overbank area is estimated as that portio n of the flow over the railroad from a point
;:00' we:st of the bridge crossing. This point was selected by aaalysis of the downstream
topography. The le:ft overbank flow was estimated using a sloping weir equatlon aad the water
surface ,~levation at cross section L given in the main channel model (see Figure 1). The flow in
the left overbank area is approximately 2,720 cfs.

;~he lirr:it of the Idt overbank floodplain on the west is defined as the flow path from the
D&RGW tracks to the Arkansas River. The left overbank area model estimates the depths of
flow. At cross section E, the water surface in the main channel is about the same as in the left
overbaak area aad the floodplains become hydrauLcally connected. Thus, it was assumed that
the majority of the flow in the left overbank area ret1lIT1S to the main chaanel. at cross section E.
A portion of the flow, however, remains in the left overbank ar'ea creating an area of shallow
floodine: to the Arkansas River.

~~he main channel flow just downstream of the D&RGW is approximately 9,840 cfs. Because of
the inac.equate capacity of the 2nd Street and 3rd Street bridge crossings to convey the main
channel flow, a portion of the flow spills to the east into the l'lght overbank area from cross
~ections H to K. The spill was quantified using the split flow option in the HEC-2 model. The
Emount of spill is about 3,880 cfs. This split flow combines with the 3,440 cfs which spills over
the railr,)ad tracks east of the main chaanel. As the combined flow in the right overbank of 7,320
c:fs moves toward the Arkansas River, a portion of the combined. flow spills back into the main
c:hannel from cross sections C to G. This split now is quantified using the split flow option in the
HEC-2 model.

At cros~, sections A aad B, the floodplains for the right overbank area aad the main channel are
hydraulically connected; aad, thus, the total flow is lIsed in the main channel model to determine
the 100-year floodplain. A general schematic of the hydraulic aaalysis is shown in Figure 1.

The Maaning's roughness coefficient is 0.045 in th<~ main channel aad raages from 0.05 to 0.15
in the left overbaak area aad 0.09 to 0.15 in the right overbank area depending on the degree of
urbanization.

If the Oak Creek floodplain were to be defined using one model which includes the main chaanel
and overbanks as opposed to three separate models, the HEC-2 program shifts the flow in the
overbanks aad main channel from one overbaak to the other. From examining the topography in
the area., the flow cannot physically shift from one overbank to aaother; thus" a single model
would not realistically defme the floodplain.

Floodway

The floodway limits will be defined based on FEMA's one foot rise criteria. If the floodway is
defined using one HEC-2 model aad the floodplain is defined using the hydra.ulic aaa1ysis for
three separate HEC-2 models, the water surface elev~.tions will not be direcily comparable to
determine the one foot rise. Also, for a floodway defined using one HEC-2. model, the HEC-2